As illustrated in FIG. 1, a lamp 10, such as may be present in an aircraft cockpit 20, that employs incandescent bulbs requires only two wires (i.e., those associated with a power-supply line 30 and return line 40) to provide power to the bulb. Any dimming is done at the front end by the use of a resistor (not shown) in series with the bulb. Typically, dimming of the lamp 10 is effected by a three-position switch associated with a control element, such as a switch box 50.
In replacing such incandescent bulbs with LEDs, a challenge lies in providing an exact back-end retrofit, without a change in the associated power-supply wiring, to allow for lamp dimming, and still maintain a constant current through the LED string at each brightness level.
LEDs require constant current. There is no guarantee of a constant current with a resistor alone. Using only a fixed constant-current circuit will prevent the lamp from being dimmable.